So, what do you need to know about SAD, other than the fact that it has the most appropriate acronym in all of medicine? Here are the basics.

What is seasonal affective disorder?SAD is a type of depression that creeps up on people every year around the time that seasons change. Most people with SAD experience symptoms during the fall-to-winter transition, but in some cases, people will get SAD in the spring or early summer.

What causes it?Though there's no definitive cause, Hillary Goldsher, PsyD, a Beverly Hills clinical psychologist, says that it's been tied to the shift in weather, which can have a psychological impact on people to differing degrees.

"It is possible that the lack of light and sun exposure has a chemical consequence on certain individuals and results in a compromise in the production of serotonin and other hormones that affect mood," said Goldsher. "It is also possible that this condition has an attitudinal component, meaning that certain people's perspectives shift and they ultimately lose a sense of strength and hope when the weather is more somber."

Your levels of vitamin D may also be affected by gloomier days, as the sun provides us with a good deal of our fix. And vitamin D deficiency has also been associated with SAD.

What are the symptoms of SAD?"SAD is a subset of major depression, so the symptoms are quite similar," explains Goldsher. Symptoms can include trouble concentrating, fatigue, irritability, greater appetite, weight gain, increased desire to be alone, increased need for sleep, and feelings of sadness, hopelessness, and helplessness.

Who's most at risk for SAD?Anyone can be affected by seasonal depression, but it's four times as common in women than in men. It generally only affects adults older than 20, and it can be more common in northern areas where it's colder and less sunny.

Goldsher also notes that people with a history of depression can be more susceptible to SAD. "If one is historically vulnerable to clinical depression when life circumstances are challenging -- in the wake of a breakup, death, or job change, for example -- [he or she] may also be vulnerable to being severely impacted when the weather changes," she explains.

Are there any treatments or preventative measures you can take?Like with any depression, talk therapy can be a helpful solution, as it gets to "the root of the melancholy" and allows you "to develop strategies to cope with the feelings."

Ideally, if you know you're prone to SAD, you'll do well to start talk therapy one to two months before the change in season -- that way, you and your psychologist can prepare and hone strategies to cope.

But don't worry if you're only just noticing it as the change sets in, as there are other treatments -- like light therapy, which would help produce serotonin and other energy-creating hormones by creating the illusion of sunlight on your face. (Note that this is different from indoor tanning beds, which raise risk of skin cancer and aren't effective in the treatment of SAD.)

The very nature of seasonal affective disorder makes people feel hopeless, but there are treatments and ways to prevent debilitating effects. If you're experiencing two or more symptoms of SAD, it's best to talk to your doctor and discuss your options.